1. Field of the Invention
The present invention relates to electronic ballasts for gas discharge lamps, particularly to ballasts wherein the load is powered by way of a series-excited parallel-loaded resonant L-C circuit.
2. Description of Prior Art
There are two predominant types of electronic ballasts for gas discharge lamps: (a) a first type may be referred-to as the parallel-resonant type and involves the use of a current-excited (i.e., parallel-excited) parallel-loaded resonant L-C circuit; and (b) a second type that may be referred-to as the series-resonant type and involves the use of a voltage-excited (i.e., series-excited) parallel-loaded resonant L-C circuit.
An example of the parallel-resonant type of electronic ballasts is described in U.S. Pat. No. 4,277,726 to Burke. An example of the series-resonant type of electronic ballasts is described in U.S. Pat. No. 4,538,095 to Nilssen.
Of these two types of electronic ballasts, the parallel-resonant type is conducive to yielding a stable easy-to-control self-oscillating inverter-type ballast; whereas the series-resonant type, although potentially simpler and more efficient, is harder to control in that it has a natural tendency to self-destruct in case the lamp load be removed and/or in case the phasing between applied voltage and resulting current were to become such that the current be leading.
To mitigate this tendency to self-destruct under no-load and/or special load conditions, various protection circuits have been developed, such as for instance described in U.S. Pat. No. 4,638,562 to Nilssen.